Switching molecules capable of changing color from one state to another under the influence of an electric field are the subject of ongoing investigation. In general, the color change occurs through a molecular conformation change that alters the degree of electron conjugation across the molecule and, thereby, its molecular orbital-induced HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) states. In a main embodiment, the conformation change occurs through field rotation of a ring or rings within the molecule. In this instance, the conjugation is broken between the rotating rings, called rotors, and ring structures that do not rotate, called stators. The rotors have electric dipoles that induce rotation within a given field. A coupling group (e.g., acetylene) between the rotor and stator elements serves as a “bearing” and conjugation bridge between the rotor and stator.
The novel molecular color switch promises a unique set of dye-like optical properties that make it ideal for applications such as electronic paper, paper-like displays, electronic books, projection displays and the like.
The concept of electronic devices based on active molecular components was proposed almost thirty years ago. Since then, hundreds of publications and numerous proposals have appeared. So far, one of the most promising architectures that will lead to highly effective and durable molecular devices is the rotor-stator configuration disclosed and claimed in above-referenced Ser. No. 10/187,720 (U.S. Pat. No. 6,701,035); Ser. No. 09/898,799 (U.S. Pat. No. 6,947,205); and U.S. Pat. No. 6,556,470.
The rotor-stator configuration of the these patent applications and patent turns ink or dye molecules into active optical-electronic devices that can be switched with an external electric field for electronic ink and other visual display applications. This type of molecule is neither oxidized nor reduced in switching from one state to the other. Furthermore, the molecule exhibits image contrast and mechanical durability commensurate with ink on paper. Although not specifically called “digital dyes” therein, that phrase is becoming associated with such ink or dye molecules.
A major challenge in the development of the molecular color switch is the need for self-assembled colorant layers on the order of 0.05 to 1.0 micrometer in thickness. Each switch molecule in the colorant layer used in print or display media must be correctly oriented with respect to the switching field and be spaced sufficiently from other molecules to allow unhindered rotation of the switching rotors. Such spacing and alignment must be repeated over a colorant thickness sufficient to achieve the optical density typical of commercial print (nominally 0.5 to 1.0 micrometer). A related challenge is to design the colorant layer for cost-effective switching voltages and addressing; Yet another challenge is to design a colorant layer that switches from a highly conjugated black to transparent state under such low switching voltages. A still further challenge is to design a means to constrain the stator sections of the molecular switch to prevent rotation with rotor rotation. The lowest conformational energy state of the typical molecular switch provides for planar rotors and stators. Un-constrained, the rotor and stator will naturally want to rotate together, vitiating any desired color change. Yet another challenge is to design a colorant layer that provides high optical density with minimized switching voltage. Still further, there is the challenge of providing a means for creating specific subtractive colors, such as cyan, magenta and yellow meeting the above rotor orientation and switching voltage needs. Further yet, there is the challenge of providing a means for bi-stable color switching enabling the colored and transparent states to remain stable indefinitely in the absence of a field. Bi-stable operation enables such applications as electronic paper and electronic books. It also provides the lowest energy alternative for display related applications since no holding voltage is required to maintain a pixel and only pixels needing to change are switched.
Additional requirements include the following: the active dye molecules in this system have to be oriented in such a way that all their rotors' dipoles should be easily aligned with the direction of the applied external electrical field (E-field). Preferably, the dipoles should be orthogonal to the field. The rotor portions of the system should be easily switched ON (in the plane) and OFF (out of the plane) with respect to the conjugated molecular system (stator portions of the system) by an external E-field. The molecular thin film between the electrodes has to be assembled in such a way that only the stator portions of the molecules will be linked together to form a 3-D network system without encroaching on the region surrounding the rotors.
To date, even though many methodologies have been developed for thin film preparation, such as Langmuir-Blodgett (L-B) technique, the Self Assembled Monolayer (SAM) method, Vapor Deposition (VD) and spin coating, etc., none of them can meet the above-discussed requirements at the same time.
For example, the L-B technique is very useful for preparing high quality thin films. Both monolayer and multi-layers thin film can be prepared using this method. However, L-B technique is only suitable for certain molecules, and these molecules must have a good hydrophilic end-group and one hydrophobic end-group connected by long alkyl chain in the middle in order to form a high quality thin film. This very strict requirement limits one greatly in terms of designing active device molecules. Furthermore, multi-layers prepared by L-B method usually have the pattern of head-head (H-H) and tail-tail (T-T) orientation, as shown in FIG. 1, which depicts a substrate 10 supporting a plurality of molecules 12 in H-H, T-T configuration, to provide a multi-layer structure 14, here, six layers 16. This particular characteristic limits one even further in terms of device preparation; one cannot make a uniform thin film with all molecules aligned in the same direction using the L-B technique.
The SAM method is another very popular way by which thin films are made. It is simple, easy and less restrictive to operate than the LB process. However, it has been used only in monolayer preparation so far, and until now, nothing has been found in literature that shows how to make multi-layer thin film.
Spin coating is another useful method for thin film preparation, and has found use in both academic research and industrial applications. This technique can be used for various thin films preparation. Adjusting both the viscosity of the coating solution and the spin speed allows one to easily control thickness of the thin film. However, this method cannot control the molecular orientation during the film preparation. It will always produce a thin film with molecules oriented randomly in all directions. Even though some researchers have attempted to use a combination of an external E-field and spin coating, the results have not been promising.
Vapor deposition techniques have been gaining popularity in recent years. Such techniques have proved to be a very useful technique in semi-conductor and other related industries. It is very good for use with materials with a low boiling point or materials with good thermal stability even at temperatures above its boiling point. Vapor deposition is usually used in metal or metal oxide deposition for obvious reasons. However, it does not work well when the materials are less thermally stable, especially at relatively high melting or boiling points. Since most large, rigid organic molecules are less thermally stable, they may undergo chemical decomposition at elevated temperatures far below their melting or boiling points. In fact, the active molecules used in the above-described color switches, such as large rigid and highly conjugated organic molecular networks, usually decompose prior to reaching their melting point (typically >200° C.).
Organized self-assembly is another possible approach, which forms the basis of the present disclosure and claims. This concept was first proposed 30 years ago. Many research groups worldwide have been working in this area since then. All work in this area so far can be classified into the following categories: (1) self-organizing polymers to achieve certain material properties; (2) self-assembling the 2-D or 3-D supra-molecular constructs via metal coordination; and (3) material assembly via hydrogen bonding or other type of intermolecular interactions.
Until now, scientists have been able to carry out organized assembly in solution, and letting those custom-designed molecules self-assemble freely in a certain organized way to form various particles (micron or sub-micron range) in solution. Undoubtedly, self-assembled materials prepared by these methods are highly organized within each individual particle. But as a whole, they are not uniform, and they are not much more than a random mixture of aggregated particles. Such self-assembly of a supra-molecular thick solid film is difficult, and the present application provides a simpler, more controllable alternate means.
Recently, an elegant way to self-assemble molecular prisms via an organometallic “clip” has been demonstrated. Combination of two tritopic pyridyl ligands with three platinum atoms containing the molecular “clip” spontaneously generates supra-molecular coordination cages with trigonal prismatic frameworks under some appropriate conditions. Several other research groups have also demonstrated similar self-assembly via metal coordination by using different noble metals, such as Ni, Os, Pd, Eu, etc.
There are several serious problems that are limiting these technologies being developed for industrial applications. One major problem is cost. The chemistry developed based on metal coordination so far relies heavily on very expensive noble metal reagents. Another problem is the stability of the reagents, most of which are air-sensitive. Extra precautions must be taken to prevent these reagents from coming into contact with oxygen, making it even less practical in large-scale manufacturing. Carrying out the assembly on a solid substrate and controlling the exact molecular orientation is apparently considered to be difficult, in that there appears to be no published information on this.
Using intermolecular hydrogen bonding is another very popular way for organized assembly. One way of molecular self-assembly through hydrogen bonding has been demonstrated. In particular, the aggregation of five molecules to form a discrete supramolecular structure by using substituted triazine and its derivatives as linking groups has been shown. However, this work is still limited in assembling soluble molecular clusters with small molecules in solution. The method cannot be used for assembly on a solid substrate, and the molecular orientation cannot be controlled by this method either.
Thus, a need remains for a technique that overcomes the above-discussed problems and permits assembling molecules of controlled orientation on solid substrates.